The technical field of this invention is the preparation of optical lenses and, in particular, the invention relates to methods and devices for preparing lens blanks to fit eyeglasses and other lens-holding receptacles.
The process of preparing optical lenses, particularly lenses for glasses and the like, normally begins with a semi-finished lens blank made from glass or plastic. Such blanks typically have a finished, polished front surface and an unfinished back surface. The necessary corrective prescription is obtained by grinding away material from the back surface, followed by polishing and edge-shaping to fit the frame or other receptacle.
During these operations, the lens blank must be accurately aligned and securely held in place. For this reason, the finished surface of the lens blank is conventionally mounted onto a "lens block" which remains joined to the lens throughout the processing steps. Blocking machines are employed to align the optical center of the lens blank and, optionally, an optical axis of the blank with a reference point or configuration on the lens block.
Conventional blocking machines employ a variety of adhesive materials (e.g., glues, pitch or low temperature fusible metal alloys) to secure the lens blank to the block. The blocks are typically hard metal or plastic and, consequently, are not well adapted to the variety of the shapes which may be presented by finished (typically convex) surface of the lens blank. Many blocking systems resort to multiple blocks having varying degrees of concavity in order to accommodate the variable curvature of lens blanks necessary to fill differing eyeglass prescriptions.
However, even when the block and blank are closely matched in terms of mating surface curvatures, complex prescriptions, such as bifocals and trifocals, require blanks that inherently have discontinuous surfaces. In these instances, conventional blocking techniques are often less than satisfactory. When the block does not securely hold the lens blank curing processing, breakage will often occur during the high speed grinding or shaving operations.
Conventional blocking techniques are also labor-intensive. In addition to the need for different blocks (having varying degrees of concavity), plastic and glass lenses can require different types of adhesives, and the surfaces must be washed to remove the adhesive not only from the finished lens but also from the block prior to its reuse in the next processing operation.
An alternative approach to blocking lenses is disclosed in U.S. Reissue Patent No. 31,897 by Johnson and involves the use of a double-sided, adhesive blocking pad which is disposed between the lens and the block. The intermediate pad serves to provide a flexible interface between the lens blank and the rigid block, which is said to permit firm bonding even when the curvatures of the blank and block do not match.
However, even when an intermediate blocking pad is used, the blocking process is still time-consuming since the alignment of the optical center and axis must be performed twice. First, the pad must be aligned with the lens blank and affixed in place. Then the lens blank and pad must together be aligned with and affixed to, the block itself. Any inaccuracy in this multiple step alignment process can result in a lens having incorrect optical refractive properties.
There exists a need for better lens blank blocking methods and devices which can quickly and accurately prepare a lens blank for grinding, edging, polishing or other shaping operations. Blocks which can be securely mounted to lens blanks, regardless of their curvature and without the need for intermediate pads, would satisfy a long felt need in the industry.